In a transmitter, a radio frequency (RF) power amplifier (PA) consumes the most energy. This is a problem for mobile devices that are battery operated, such as cellular telephones, and other wireless devices. The RF PA affects the performance in terms of efficiency, linearity, and output power. The efficiency of the PA for converting the battery power to RF power of the signal determines to a great extent the length of time the device can be used before recharging, if at all possible.
The efficiency of the PA is dependent on factors including, but not limited to, semiconductor technologies, amplifier architecture, mode of operation, and an impedance matching network. The performance of PA is also affected by manufacturing and packaging procedures. The instantaneous efficiency of the PA is closely related to the signal amplitude. It is known that the RF PA operates most efficiently when the input signal has a high power mode, which is close to the device saturated power. In general, the efficiency of the PA degrades with a decrease of the input signal power level. Consequently, the conventional PA normally operates inefficiently at a middle power mode, and very inefficiently at a low power mode, in comparison with high power mode operation.
This inefficiency is more severe in the smart phones for third and fourth generation (3G 4G) mobile communication that use a varying amplitude modulated RF signal. Spectrum-efficient modulated signal, such like Wideband Code Division Multiple Access (WCDMA), Long Term Evolution (LTE) and LTE-Advanced signals, normally have high peak-to-average power ratio (PAPR). Typically, the probability density function of those signals indicates that the highest probability happens at the average power levels, approximately 7-10 dB back-off with respect to the peak power level. This implies that the RF PA operates inefficiently most of the time under these scenarios. The output power level of the power amplifier depends also on a distance between the mobile device and a base transceiver station (BTS), the transmission environment, and impedance mismatch conditions.
Hence, the enhancement of RF power amplifier efficiency at middle and low power mode is important. There are several conventional approaches for increasing the efficiencies of power amplifiers with multiple power modes.
A Doherty amplifier improves efficiency at low and middle power modes, and is widely used for wireless communications, especially at the base station. The Doherty amplifier uses a main amplifier and an auxiliary amplifier. The operation principle of Doherty amplifier is based on a load modulation from the auxiliary amplifier to the main amplifier at different power levels. Although the Doherty amplifier is capable of offering relative high efficiency at peak power level and at average power level, it is not very suitable for mobile devices because of its large size, and inherent limited bandwidth.
Another enhancement approach is envelope tracking (ET), which dynamically adapts the supply voltage to the envelope of the input RF signal. ET is theoretically able to offer high efficiency amplification independent of power level. However, the main limitation and challenges of applying ET to mobile devices come from extra circuit modules (power modulator), and challenges of envelope signal alignment, and cost.
As shown in FIG. 1, a stage-bypass technique, which selects different branches or combinations of power amplifiers according to the actual power mode to achieve efficient operation, is also commonly used in the mobile telephones. For the high power mode, the input RF signal 101 is amplified by both PA1 110 and PA2 120 to the output RF signal 102. For the lower power mode, PA2 is bypassed and the RF signal is only amplified by PA1. The signal path is selected by the switches 115 and 125. However, disadvantages of the stage-bypass technique include the complexity due to the number of amplifiers. The design becomes more complicated for multi-band PA modules, where the PAs operate work at different frequency hands.
Load modulation is another method to improve the efficiency at low output power levels. Given that the power amplifier operation and its efficiency is significantly influenced by the load impedances for transistor at certain bias conditions, the load conditions can be dynamically adjusted according to the power modes at which the amplifier actually operate to obtain an efficient operation at different power levels.
Only one transistor is needed for the amplifier implementation. A adjustable impedance matching network for load modulation technique can be reconfigured by selecting different matching element, for example inductors, capacitors, and transmission lines, or combinations using switching elements, for example using gallium arsenide (GaAs) pseudomorphic-high electron mobility transistor (pHEMT) switcher.
Alternatively, the impedance matching network can be tuned with a varactor using control signals, such as voltage applied between two terminals, to change its reactance. However, the extra complexities, power loss, reliabilities of the switchable and/or tunable elements are a disadvantage of the load modulation approach, when the matching network is not properly designed.
Hence, there is a need for improving power amplification for multiple power mode power amplifiers, particularly in battery operated wireless devices.